danesherbs/tdd-skeleton

The TDD loop: Red, Green, Refactor.

Write a failing test.
Write the minimal amount of code for the test to pass.
Refactor (if needed).

Advantages

Tests give confidence when refactoring since they act as a safety harness.
Brittle tests are quickly found in the refactoring step of the TDD loop.
Following the TDD loop results in an emergent design, which is often preferred over a plan-then-implement design.

Test structure

Use the Given-When-Then structure

Improves the readability of test cases, especially when there are many.

❌

def test_dog_makes_woof_sound():
  assertTrue("woof" in Dog().bark().sounds())

✅

def test_dog_makes_woof_sound():
  # Given
  dog = Dog()
  
  # When
  dog.bark()
  
  # Then
  assertTrue("woof" in dog.sounds())

Test one behaviour at a time

Allows you to test behaviours independently of one another.

Makes debugging easier when test cases are failing.

❌

def test_dog_makes_sounds():
  # Given
  dog = Dog()
  
  # When
  dog.bark()
  dog.sniff()
  
  # Then
  assertTrue("woof" in dog.sounds())
  assertTrue("sniff" in dog.sounds())

✅

def test_dog_makes_woof_sound():
  # Given
  dog = Dog()
  
  # When
  dog.bark()
  
  # Then
  assertTrue("woof" in dog.sounds())
  

def test_dog_makes_sniff_sound():
  # Given
  dog = Dog()
  
  # When
  dog.sniff()
  
  # Then
  assertTrue("sniff" in dog.sounds())

Keep tests independent

Independent tests are easier to reason about.

❌

dog = Dog()

def test_dog_makes_woof_sound():
  # When
  dog.bark()
  
  # Then
  assertEqual(dog.sounds(), ["woof"])

def test_dog_makes_sniff_sound():
  # When
  dog.sniff()
  
  # Then
  assertEqual(dog.sounds(), ["sniff"])  # Failure: expected  ["sniff"] but got ["woof", "sniff"]

✅

def test_dog_makes_woof_sound():
  # Given
  dog = Dog()
  
  # When
  dog.bark()
  
  # Then
  assertEqual(dog.sounds(), ["woof"])
  

def test_dog_makes_sniff_sound():
  # Given
  dog = Dog()
  
  # When
  dog.sniff()
  
  # Then
  assertEqual(dog.sounds(), ["sniff"])